LONDON (Reuters) - Rich countries must commit to cutting
carbon emissions by 80 percent by 2050 and developing nations
must agree that by 2020 they too will set their own targets,
leading economist Nicholas Stern said on Wednesday.

He said the only way the world could defeat the climate
crisis was by ensuring that global carbon emissions peaked
within 15 years, were then halved from 1990 levels to 20
billion tonnes a year by 2050, and cut to 10 billion
thereafter.

"There is a real hurry for this. The developed world must
lead by example," Stern told a meeting to publish his latest
work on global warming, "Key Elements of a Global Deal on
Climate Change."

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The global carbon market had to be expanded and improved,
there had to be massive investment in research and development
in low carbon technologies, and rich nations had to bear the
brunt and help the poorer world leapfrog into a low carbon era.

Stern said the developing world, where emissions are
booming as economies grow, should be given time to prepare to
sign up to caps and cuts but that time should have a strict
limit and by 2020 they too should be reducing emissions.

Stern, a former British Treasury economist whose seminal
work 18 months ago on the economics of climate change
galvanized the international agenda, said the emission target
was based on the goal of halting the temperature rise to two
degrees Celsius above pre-industrial levels.

That in turn meant achieving global average carbon
emissions of just two tonnes per head -- 20 billion tonnes
divided by the anticipated world population of nine billion
people -- from the current average of seven tonnes per head, he
said.

"Everything flows from the figures. That is the simplicity
of the argument. If you buy into stabilization at 500 parts per
million (atmospheric carbon -- equivalent to two degrees rise)
the rest is arithmetic," Stern told an audience at the London
School of Economics.

As emissions in the United States already stood at 20
tonnes per head, with those in Europe and Japan between 10 and
12 tonnes, that meant the bulk of the efforts had to come from
the rich world.

But even China, whose economy is growing at 10 percent a
year and which is building a coal-fired power station a week,
was already emitting five tonnes of carbon a head and India was
close to two tonnes and would soon exceed that.

That meant that they too would have to slow, halt and
reverse their emissions.